Simplify preMountItems frame time check (#46347)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/46347

Calculate a frame deadline, and compare the current time against that.

This will also allow us to make frame timing more dynamic in the future, based on the display's frame rate.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D62213710

fbshipit-source-id: 1c7fc4b67d08c1eda4f3b9612a4506b33c44f626